1. Field of the Invention
The present invention relates to a thermoelectric material consisting of a substance having a new and novel structure and/or configuration and a method of manufacturing the thermoelectric material.
2. Description of the Related Art
Heretofore, chalcogenide system materials such as Bi2Te3, PbTe and the like, Si—Ge system mixed crystal materials, or the like have been taken up as thermoelectric materials. Thermoelectric figure of merit Z that determines the efficiency of thermoelectric conversion in a thermoelectric material is expressed by the following equation using thermal conductivity κ, electrical resistivity ρ and Seebeck coefficient (thermally generated electromotive force per unit temperature difference) S.Z=S2/κρ
The figure of merit Z has a dimension that is the reciprocal of temperature T, and there are many cases that the product ZT of the figure of merit Z and the temperature T is used as the dimensionless or zero-dimensional thermoelectric figure of merit. From the above equation, it may be said that a material having good thermoelectric conversion efficiency is a substance which is large in Seebeck coefficient S as well as is small in both electrical resistivity ρ and thermal conductivity κ.
Incidentally, almost all of thermoelectric materials that have been practically used until now have been semiconductors, and yet have been limited to degenerate semiconductors having high mobility as cited above. This is due to the following reasons.
In case conduction will occur by band electrons (or holes), since κ, ρ and S all depend upon density of carriers (carrier density) n, Z is a function of n and comes to the maximum in the order of n=n0=1019 cm−3. This corresponds to the carrier density of a semiconductor having electrons or holes in comparatively high density (since its energy distribution forms degenerated Fermi distribution, it is called a degenerate semiconductor).
On the other hand, heat is conveyed not only by conduction electrons but also by lattice vibration (phonon), and the thermal conductivity κ is expressed by the following equation as the sum of contribution of electron and contribution of lattice.κ=κelectronic+κlatticeκelectronic is determined depending upon the carrier density, but κlattice depends upon elements constituting the material and/or structure of the material.
Accordingly, in order to obtain small ρ and κ under the optimum carrier density n0, there is no alternative but to increase the mobility and to reduce the thermal conductivity κlattice conveyed by phonon. For that reason, all the prior thermoelectric materials were degenerate semiconductors each having high mobility, and a contrivance has been done, which intends to decrease mean free path of phonon thereby to reduce κlattice by using heavy elements such as Bi, Te, Pb and/or by making the materials mixed crystallization.
However, a design for substance based on the prior crystalline semiconductor materials is of less degree of freedom in controlling individually the electrical nature or property and the thermal nature or property of a material, and in practice, if an element or elements constituting a crystal and structure thereof are determined, values of κ, ρ and S are substantially bounded to known ones. Accordingly, it is difficult to attain a design for substance in which S, ρ and κlattice can be individually controlled to greatly improve the efficiency of thermoelectric conversion, and it has not been materialized to put a thermoelectric material having large ZT, such as beyond 1.5, to practical use.